1. Field of the Invention
The present invention relates to a configuration of water drain holes formed in an exhaust duct for exhausting gases, such as air.
2. Description of the Related Art
Heretofore, electric vehicles, hybrid electric vehicles, and fuel cell vehicles were equipped with a battery pack for storing electric power to supply a drive motor. The battery pack includes an exhaust duct for exhausting air that blows in via an air inlet for cooling a battery stack formed by combining multiple battery modules.
FIG. 7 shows a case structure of a conventional battery pack. A battery case, which is a case for a battery pack 100, includes an upper case 12 for covering the upper part of the battery stack and a lower case 14 for covering the lower part. Built into the battery pack 100 is a battery stack having battery modules formed from one or multiple cells, such as nickel-metal hydride battery or lithium ion battery, having cooling passages located between them in parallel, assembled by binding between end members located at both ends, and having the battery modules electrically connected in series. Furthermore, a cooling fan is provided inside the battery pack 100 at one end in the longitudinal direction (x direction) of the battery pack 100. An air inlet 16 is provided on one side of the battery pack 100 and an outlet 18 is provided on the other side of the battery pack 100 near the cooling fan. When the cooling fan is rotationally driven, air within the passenger compartment is taken in from the air inlet 16. The cooling air that is taken in is supplied to a chamber space formed between the battery stack and the lower case 14, flows from the bottom of the battery stack to the top via cooling passages between battery modules, passes a chamber space formed between the battery stack and the upper case 12, and further reaches an exhaust duct 150 via the cooling fan and is exhausted to the outside from the outlet 18.
When the cooling air is compressed by the cooling fan, the water vapor contained in the cooling air liquefies. Namely, the cooling air exhausted by the cooling fan contains moisture. To exhaust the liquid, such as moisture, that collects in the exhaust duct 150 to outside the battery pack 100, water drain holes are formed in the bottom face of the exhaust duct 150 and in the lower case 14.
FIG. 8 shows a perspective view of the conventional exhaust duct 150. Furthermore, FIG. 9 shows a cross section of part of the battery pack 100 as viewed from the direction of the arrow when sliced in a vertical direction along line A-A of FIG. 7. As shown in FIG. 9, in the prior art, a water drain hole 200b in the lower case 14 is formed immediately below a water drain hole 200a in the exhaust duct 150 and liquid that collects within the exhaust duct 150 is exhausted to outside the battery pack 100 via the water drain hole 200a and the water drain hole 200b. 
Aside from the liquid that is exhausted via the water drain hole 200a and the water drain hole 200b, leakage of part of the cooling air may cause turbulence in the cooling air and generate noise near the water drain holes.
Especially, in the case of a battery pack mounted in a vehicle, the noise generated by the battery pack may reach the passenger compartment. However, in recent years, the demand is high for quieter passenger compartments to accompany the higher quality of vehicles so that the demand is also high for the reduction in noise generated in the battery pack mounted in the vehicle.
Japanese Patent Laid-Open Publication No. Sho 61-64523 and Japanese Patent Laid-Open Publication No. 2004-243855 disclose a duct formed with a hole for exhausting water to the outside.